The demand for small-sized and large-capacity nonvolatile memory devices (memory) rapidly increases these days under a circumstance in which small-sized portable devices are becoming widespread toward the realization of a ubiquitous society. In particular, NAND flash memory and small-sized HDD (hard disk drive) have made rapid progress in recording density and form a big market.
However, the NAND flash memory and the small-sized HDD are considered to reach the limit of recording density in the future.
The NAND flash memory may reach the limit of miniaturization due to short channel effects because the memory unit uses a transistor. The “short channel effects” are phenomena that occur due to a short distance between the source portion and the drain portion caused by the miniaturization of the device, specifically such as an increased leak current between the source and the drain. Furthermore, there is an increase in manufacturing cost due to the reduction of the minimum line width.
The small-sized HDD may reach the limit of tracking accuracy (the accuracy of adjusting a head to the correct position of a track on the HDD).
Accordingly, developing new technology that breaks the limit of recording density is desirable.
As such new technology, a phase-change nonvolatile memory device (phase-change memory) (PCRAM: phase-change random access memory) is investigated. The phase-change nonvolatile memory device is a nonvolatile memory device using the property that a semiconductor film (phase-change film) changes between a crystal state and an amorphous state by applying an electric field pulse to the phase-change film. By reversibly changing the phase-change film between a high resistance state (amorphous state, ON) and a low resistance state (crystal state, OFF), information is stored so that the information can be rewritten and may not be erased even if the power supply is turned off. Nonvolatile properties are obtained because both the high resistance state and the low resistance state of the phase-change film are stable. The phase-change nonvolatile memory device does not have the problems described above which may occur in the NAND flash memory or the small-sized HDD, and seems to be superior to the above two devices in high speed properties and miniaturization capability.
Here, a relatively large current flows in the phase-change nonvolatile memory device during operation, in particular at the time of erasing (reset). Therefore, it is required that the operating current be reduced.
As conventional technology in regard to this, there is a semiconductor integrated circuit device in which an alloy film (GST film) of germanium (Ge), antimony (Sb), and tellurium (Te) is used as a memory element and an ion of nitrogen, oxygen, or carbon is doped. For example, a region implanted with high concentration oxygen ions exhibits almost insulating properties and a pathway of a current for rewriting and signal transmission is limited to a low concentration ion implantation region (JP-A 2006-156886 (Kokai)).